Fluorine-containing rubbers are excellent in characteristics such as fuel impermeability, sliding property, heat resistance, chemical resistance, weather resistance and electrical properties, and are used in various fields such as automobiles, industrial machinery, office automation equipment and electrical and electronic equipment.
For example, in the field of automobiles, fluorine-containing rubbers are used as a sealing material for an engine and peripheral equipment thereof, automatic transmission, fuel system and peripheral equipment thereof and the like. However as a result of recent strict control of environmental regulations, regulations on SHED (Sealed Housing for Evaporative Determination) have been made more rigorous, and especially development of rubber materials for fuel system having fuel impermeability is demanded. Rubber materials for fuel system are required to have various characteristics such as processability, oil resistance and cold resistance in addition to fuel impermeability, and fluorine-containing rubbers are not satisfactory enough to meet such requirements because they are inferior in cold resistance though fuel impermeability is excellent.
Further in the case of engines for cars, strict conditions for use thereof are demanded, for example, injection of high pressure air-fuel mixed gas directly into a combustion chamber is carried out for the purposes of increasing specific fuel consumption and reducing exhausted carbon dioxide. As a result, when a fluorine-containing rubber being inferior in cold resistance is used for high pressure sealing, there is a problem that when a temperature is lower than a glass transition temperature, rubber elasticity is lost and sealing of a high pressure fuel gas becomes difficult.
In order to solve this problem, various studies have been made with respect to composite materials prepared by combining a fluorine-containing rubber being excellent in fuel impermeability and a silicone rubber being excellent in cold resistance. Those two kinds of rubbers have property of making up for disadvantages of each other, and therefore it is expected that a new material can be provided if they are well combined in the form of a blend, an alloy or the like.
For example, JP-A-1-198646 discloses a rubber composition obtained by dynamically crosslinking a silicone rubber (I), an elastomer (II) which is not crosslinked only with an organic peroxide, a co-crosslinkable elastomer (III) which is co-crosslinkable with the silicone rubber (I) with an organic peroxide and co-crosslinkable with the elastomer (II), and an organic peroxide. In the case of the rubber composition disclosed in JP-A-1-198646, by crosslinking the silicone rubber (I) and the elastomer (III) to be added as case demands simultaneously when mixing, a system is fixed while a satisfactory dispersed state is maintained, and then the elastomer (II) is crosslinked at molding. In this method, since the crosslinking of the silicone rubber is carried out during mixing, a shape of the silicone rubber hardly becomes spherical, which makes it difficult to maintain continuity of the matrix phase and enhance fuel impermeability. In addition, studies are not made concretely by using cold resistant fluorine-containing rubbers.
Also JP-A-1-242650 discloses a fluorine-containing rubber composition comprising a fluorine-containing rubber and crosslinked silicone rubber particles having an average particle size of not more than 10 μm. However, there is no disclosure as to crosslinked fluorine-containing silicone rubber particles. In the method disclosed therein, the crosslinked silicone rubber particles do not have a functional group, interlayer adhesion is not satisfactory and a sufficient tensile strength is not obtained. In addition, studies are not made concretely by using cold resistant fluorine-containing rubbers.
As mentioned above, there have been no fluorine-containing composite materials having both of improved fuel impermeability and improved cold resistance.